Balanced amplifier



April 6, 1948. D. BLITZ 2,438,960

BALANCED AMPLIFIER Filed larch 30, 1945 1111' Q) [Q i Q earn/r INVENTOR. D

BY E

Patented Apr. 6, 1948 2,438,960 Bamucan AMPLIFIER Daniel Blitz, Princeton, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application March so, 1945, Serial No. 585,603

6 Claims. (01. 179-171) This invention relates generally to direct and alternating current amplifiers and more particularly to balanced amplifiers which are compensated for variations in power supply voltage.

Direct current amplifiers usually are responsive to changes in power supply voltage. These changes alter the gain of the amplifier and disturb the zero setting. In both A.-C. and D.-C. amplifiers requiring a high degree of gain stability, it is customary to provide a regulated power supply source. Such regulated power supply sources are complicated, expensive and often are incapable of providing the desired gain stability.

The instant invention is an improvement upon the device disclosed and claimed in U. S. Patent 2,310,342, granted to Maurice Artzt on February 9, 1943,-and assignedto the same assignee as the instant application. The invention herein will be explained first by reference to the device described in said patent and then by reference to various modifications and improvements thereof to provide the desired gain stability,

Among the objects of the invention is to provide stabilized means for amplifying direct currents. Another object of the invention is to provide A.-C. and D.-C. amplifying means in which the D.-C. level is substantially independent of variations in power source voltage. An additional object is to provide stabilized amplifying means in which the gain may be varied without disturbing the D.-C. zero input setting. A further object is to provide current amplifying means including self-regulation of energizing power source voltages. An additional object of the invention is to provide an improved gainstabilized amplifier in which the output voltages are stabilized with respect to one terminal of the energizing voltage source. A further object is to provide an improved current amplifier having a feedback circuit for making the amplifier signal output substantially independent of energizing voltage fluctuations.

The invention will be described by referencev to the accompanying drawing of which Figure 1 is a schematic circuit diagram or a prior art am plifying system of which the instant invention is an improvement, Figure 2 is a schematic circuit diagram of a first embodiment of the invention and Figure 3 is a schematic circuit diagram of a second embodiment of the invention. Similar reference characters are applied to similar elements throughout the drawing.

Referring to Figure 1, a power source, not shown, is connected to power terminals I, 3, which are also connected to the end terminals z of a pair of serially-connected resistors 5, I, having equal resistance values and having a common intermediate terminal 9. An anode resistor I I, the anode-cathode circuit of an output triode I3, an intermediate anode resistor I5, the anodecathode circuit of an input triode I1, and a cathode resistor I9 are serially-connected, in the order named, between the power source terminals I and 3. If desired, the negative power source terminal 3 may be grounded. An output impedance II is connected between the cathode of the output triode I9 and the intermediate terminal 9 on the voltage divider 5, 1. Output signals are derived from output terminals 23, coupled to the end terminal and an adjustable tap on the output impedance 2I. The control electrode of the output triode I3 is connected to the anode of the input triode I'I, whereby the tubes .are coupled in cascade. Input signals are applied to the input terminals 25, 21 which are connected, respectively, to the grid of the input triode I1 and to ground. A grid resistor 29 is connected in parallel with the input terminals 25, 21.

The operation of the circuit is as follows:

Whatever the voltage of the power source may be,

it is divided equally across the voltage dividing resistors 5 and I. Preferably the voltage of the power source is selected to beequal to twice the rated voltage of either tube. Resistors I5, I9 are normally equal, and of a value which permits normal anode current to fiow through the tubes for class A amplifier operation. It follows that the current through the cathode resistor I9 will equal the current through the intermediate resistor I5 and will be of the proper value to bias the output amplifier tube I3 for correct class A operation. Furthermore, if the resistance of the anode resistor If is selected to be equal to the sum of the resistances l5 and I9, the anode currentthrough the output tube I3 will equal that through the input tube II. Therefore, the voltage drop through the input tube I1 and the resistors I5 and I9 will equal the voltage drop across the voltage dividing resistor 'I. Also, the

"voltage drop through the output tube I3 and the anode resistor II will equal the voltage drop across the voltage dividing resistor 5. Since the currents through the output impedance 2| (in the absence of an input signal), are equal and opposite, there will be zero output voltage across said output impedance. ,Thus, the output voltage applied to the grid 01 the input amplifier tube II, the anode current 01 the tube will increase, and this will increase the voltage drop in the resistors I and I9, and across the output impedance 2|. The current in the cathode resistor I9 will be degenerative, and the final change in bias across the cathode resistor I9 may be one volt less a hall volt, or a half volt positive. A similar voltage change across the intermediate resistor I5 will be in the opposite direction and will decrease the current flowing through the anode resistor II and through the ouput impedance 2|. Since, in the presence of an input voltage, the anode current or input tube I'I increases the voltage drop across the output impedance 2|, and the anode current of the output tube I3 decreases the voltage drop across said output impedance, it follows that a negative ouput voltage will be developed across the output impedance.

It the input signal to the input tube I! had been assumed to be negative, the anode current of the tube would have decreased, and the anode current of the output tube I3 would have increased, thereby making the output voltage across the output impedance positive. 'By setting the value of the cathode resistor II! to a value which makes the potential across the output impedance 2| a zero value with no input signal, the system will be balanced, the output voltage will be affected only b the input signal, and the gain will be constant over a relatively wide variation of supply voltage. The gain of the amplifier may be varied by means of the adjustable tap on the output impedance 2|.

The device thus described substantially balances the output signal voltages with respect to the potential of the intermediate terminal 9 on the voltage divider 5, I and substantially compensates for variations in the power source over a substantially wide range.

The improved circuit employing the instant invention, illustrated in Figure 2, is essentially similar to the circuit of Figure 1, with the exception that resistors 5, I and 2| are omitted and the input signal applied to the input terminals 25, 21 is applied between the grid of the tube I1 and the common terminal 3| of a feedback net-- work including resistors 33 and 35. terminals of the feedback network 33, 35 are connected to the positive terminal of the anode resistor I I and to ground, respectively. Output signals are taken from terminals 23 connected to the cathode of the output tube I3 and the negative power supply terminal 3, If the-values of the-resistors 33, 35 are selected so that the ratio of the resistance of the resistor 35 to the sumof the resistances of the resistors 33 and 35 is equal substantially to the reciprocal of twice the amplifier gain, output signals will be stabilized with respect to the grounded negative terminal of the power supply. This condition obtains since feedback voltages from the positive terminal of the anode resistor II applied to the grid circuit of the input tube I! are of the correct amplitude and phase to neutralize the eflect of variations in anode voltage, so that the D.-C. outputlevel remains substantially constant in the presence of energizing voltage variations.

A bias voltage, supplied by a battery 31 connected in series between the cathode resistor I9 and ground, maintains the cathode of the input tube I! at a positive potential with respect to ground. This bias voltage should be selected to be substantially the same as the quiescent voltage on the grid of the tube resulting from the feed- The end back circuit 33, 35. A resistor R1. may be connected between the positive terminal I oi the Power supply and the ungrounded'output terminal it the output impedance is other than a relatively high value. Gain control may be provided by an adjustable tap 39 on the shunt resistor 29, which is connected across the input terminals 25, 21,

The circuit of Figure 3 is similar to the circuit of Figure 2 with the exception that the common terminal 3| oi the feedback resistor network 33, 35 is grounded. Output signals are derived from the cathode of the output tube I3 and the negative terminal 3 oi? the power supp y. The p ation oi the circuit of Figure 3 is substantially identical to that of the circuit of Figure 2 in that the output signals are stabilized with respect to the negative terminal 3 of the power source. In both circuits, output signals are compensated for energizing voltage variations, and the output is stabilized with respect to the reference level of the input signals.

Thus, the invention described comprises a selfstabilizing A.-C or D.-C amplifier, wherein the D.-C, level is substantially independent of power source variations as a. result of a novel balancing circuit. The balancing characteristics tend to neutralize hum or like disturbances in the power source, thus permitting cheaper and more efiicient rectifier and filter systems than may'cuse tomarily be employed as a power source for high fidelity amplifier systems. The novel feedback circuit also balances-the amplifier output signals with respect to selected points of circuit reference potential,

I claim as my invention:

1. In a balanced amplifier including terminals for a source of unidirectional voltage, input and output electron discharge devices provided with cathode-anode circuits including three anode circuit resistors, said resistors and said anode-cathode circuits of said devices being connected in series across said source terminals and said discharge device being coupled in cascade, and output means connected between the cathode of said output device and the negative terminal of said voltage source, the improvement comprising a feedback circuit including impedance means having end terminals connected to said source terminals and an intermediate terminal, and signal supply means connected between said intermediate terminal and the input circuit of said input device.

2. In a balanced amplifier including connections for a source of unidirectional voltage, input and output electron discharge devices provided with cathode-anode circuits including three anode circuit resistors, one of said resistors being connected in series between one of said source connections and the anode of said output device, a second of said resistors being connected in series between'the other of said source connec tions and the cathode of said input device, and

- and an intermediate terminal, and signal supply means connected between said intermediate terminal andthe input circuit of said input device aisaeco for stabilizing the output of said amplifier to compensate forenergizing voltage source variations.

3. In a balanced amplifier including connections for a source of unidirectional voltage, input and output electron discharge devices provided with cathode-anode circuits including three anode circuit resistors, one of said resistors being connected in series between one of said source connections and the anode of said output device, a second of said resistors being connected in series between the other of said source connections and the cathode of said input device, and the third of said resistors being connected in series between the vanode of said input device and the cathode of said output device, said output device having its input circuit connected across said third resistor, and an output circuit connected between the cathode of said output device and said other source connection, the improvement comprising a feedback circuit ineluding a pair of serially-connected resistors connected between said source connections, and signal supply means connected between the Junction of said feedback resistors and the input'circult of said input device for stabilizing the output of said amplifier to compensate for energizing said junction of said serially-connected imped ances is a point of reference potential.

6. Apparatus as claimed in claim 3 wherein said connection of said feedback circuit to said other source connection is a point of reference potential.

DANIEL BH'IZ. 

